Stabilization of polyunsaturates

ABSTRACT

Polyunsaturated oils such as triply or higher ethylenically unsaturated organic materials are stabilized against ambient temperature atmospheric oxidation by the addition of an essential oil such as thyme oil, oregano oil, pepper oil or clove oil.

The invention relates to the stabilisation of polyunsaturates, that isto say triply or more highly ethylenically unsaturated organicmaterials, and especially to essential fatty acids of this class.

Essential fatty acids (EFAS) are nutrients, which are essential forhealth and well being, and like vitamins must be ingested, since theycannot be synthesised in the body. They are found in vegetable seedoils, fish oils and other natural sources and are readily susceptible tospoilage by mild oxidation conditions such as by contact withatmospheric oxygen at ambient temperature. Antioxidants are required toprotect these sensitive and labile materials.

Most vegetable oils which contain an n-6 EFA contain only linoleic acidwhich has two double bonds. However, some oils of vegetable, fungal oralgal origin contain other, more unsaturated fatty acids such asgamma-linolenic acid (18:3 n-6, GLA), dihomo-gamma-linolenic acid (20:3n-6, DGLA) and arachidonic acid (20:4 n-6, AA). Since susceptibility tooxidation is related to the number of double bonds, oils or purifiedcompounds containing GLA. DGLA, or AA are highly susceptible tooxidation.

Oils from various sources frequently contain highly unsaturated fattyacids of the n-3 series, such as eicosapentaenoic acid (EPA. 20:5 n-3),docosapentaenoic-acid (DPA. 22:5 n-3) and docosahexaenoic acid (DHA,22:6 n-3). These fatty acids are particularly susceptible to oxidation.

Most oils of natural occurrence, especially vegetable seed oils alreadycontain antioxidants, most commonly phenolic compounds such astocopherols. In some cases however, especially when the levels ofnatural antioxidants have been reduced by refining, by purifying or byother processing, it is advantageous to supplement residual antioxidantsby the addition of like compounds of natural or synthetic origin. Thisis particularly important with natural vegetable, fungal or algal oilscontaining polyunsaturated fatty acids such as GLA and/or AA, and withmarine oils containing EPA, DPA and DHA.

A quite different class of oils is formed by essential oils, extractedfrom a wide range of culinary and medicinal plants. They have been knownfrom antiquity to possess important biological effects, and areprincipally known for their antifungal and antibacterial properties.Their defining characteristics are steam distillability andlipophilicity, and they are, usually, odorous mixtures of esters,aldehydes, alcohols, ketones and mono- and sesqui-terpenes of which theterpenes are particularly significant.

Recently it has been reported that whole essential oils have antioxidanteffects in vivo (Deans, Noble & Svoboda, 1992). It has also beenreported that specific methanolic extracts from some herbs can stabiliselard at 75° C. (Economou, Oreopoulou, Thomopoulas, 1991). Such extractscorrespond only in part to essential oils, and the paper is primarilyconcerned in any case with stability in high temperature use, as incooking oils. Furthermore it does not concern the triply and more highlyethylenically unsaturated fatty acids that are particularly valuablecomponents in many natural oils and are highly susceptible to oxidationeven at room temperature.

Another publication, in JP 5-255692 (Furn Seiyu KK), is of fats showinglow absorption from the gut, in which the fatty acids comprisepolyunsaturates, and among possible antioxidants is rosemary oil. Thisis no doubt one of the many processed and fractionated extractsavailable for use in foods and cooking oils. A still further report, byToyama et al Bull.Jap.Soc. Sci. Fisheries 39 891-898(1973) is offractionated citrus fruit oils containing above 90% terpenes beingfairly effective in room temperature storage tests in flat fish off.

These publications, and for example others using citrus fruit oils forflavour masking in fish oil products, are unspecific. We have nowhowever found that for the particular problem of long-term,ambient-temperature stabilisation of triply and more highly unsaturatedmaterials, especially in pharmaceuticals where particular and individualmaterials are often required to be preserved for their specific effectsand stability in terms of years may be required, there is particularvalue in the use of whole essential oils. As extracted, desirably fromthe vegetative parts of the oil-containing plant rather than fruits, thesteam distillates form a natural and highly effective antioxidantsystem.

Accordingly, the invention provides a method of long-termambient-temperature stabilisation of polyunsaturates, that is to saytriply or more highly ethylenically unsaturated organic materials, andespecially essential fatty acids, susceptible to atmospheric oxidation,which method comprises adding to the polyunsaturate at least one wholeessential oil.

The invention further provides a composition constituting apharmaceutical for application by any route, a topical preparation forthe care or treatment of normal or diseased skin, or a nutritionalsupplement or a special food for human or veterinary use, whichcomposition comprises a polyunsaturate stabilised according to themethod set out above.

Valuable essential oils include those extracted from thyme, oregano,pepper and clove. Whole thyme oil, for example, can delayambient-temperature oxidation of polyunsaturated fatty acids or theiresters such as glyceride oils to give extended spoilage time underordinary commercial conditions. Furthermore, the presence of endogenousprimary antioxidant tocopherols and/or tocotrienols can further enhancethe protective effect of the added essential oils. This has been foundto be true also for added primary antioxidants, such as tocopherol,tocotrienol, ascorbic acid and derivatives, especially fat solublederivatives such as ascorbyl palmitate.

The essential oils contain a wide range of volatile compounds, such asmonoterpenes and sesquiterpenes. Although specific extracts such asrosmaric acid and apigenin have been identified as antioxidants, thepresent invention discloses for the first time the use of the wholeessential oil as such in stabilising polyunsaturated essential fattyacids. The present invention is further distinguished from previousproposals concerned with glyceride oil stabilisation in that it isprimarily concerned with long term stabilisation at ambient temperaturesnot for example with hot cooking oils, and with polyunsaturated fattyacids and their esters rather than more easily stabilised materials oflower degrees of unsaturation.

The stability of edible oils is usually judged by means of anaccelerated test, conducted at 100° C. or above, that seeks to effectrapid oxidation. For oils designed for frying, baking or other culinarypurposes this is realistic. However, if oils are specifically forstorage and consumption at ambient temperatures, such as for nutritionalsupplements or for medicinal or pharmaceutical applications, such testsare not realistic. Not only does the oxidation mechanism change incharacter as the temperature is raised, but polyunsaturated acids in anycase undergo chemical ranges at elevated temperatures that may haveadverse effects on their biological properties. Accordingly, an,accelerated room temperature test has been developed which isappropriate to the evaluation of edible oils particularly susceptible tooxidation by atmospheric oxygen.

In this test, which is called the open dish test, oil is exposed in athin layer to atmospheric oxygen at room or other temperature, usuallyin a Petri dish of 10 cm diameter. The rate of spoilage is measured bydetermining peroxide values periodically, usually over a period ofseveral days. The time needed for the peroxide value to treble itsoriginal value is a useful quantitative comparative measure of rates ofspoilage, and is known as T₃. It allows ready identification ofeffective amounts of the essential oils and primary antioxidants whenused. Suitably the essential oil is added in an amount of the or eachoil of from 0.001 to 10% by weight related to the polyunsaturate,preferably from 0.01 to 2%, and most preferably from 0.05 to 1% and theprimary antioxidant is added in an amount of from 0.001 to 0.1% byweight related to the polyunsaturate, preferably 0.01 to 0.02%.

The invention is further illustrated by the following Examples.

EXAMPLES Example 1

Freshly extracted unrefined evening primrose oil with a peroxide value(PV) of 8.2 oxidised progressively in the open dish test with a T₃ of 25days. The addition of 0.05% of thyme oil from a commercial sourceextended the T₃ to 62 days.

Example 2

Fully refined evening primrose oil with an initial PV of 2.8, whichcontained only 100 ppm of residual endogenous total tocopherols, had aPV of 5.2 and a T₃ of 2 days in the open dish test. The addition of 0.1%oregano oil reduced the PV to 1.3 after 3 days and gave a T₃ of 11 days.

Example 3

Freshly extracted unrefined evening primrose oil with a peroxide value(PV) of 10.0 oxidised progressively in the open dish test with a T₃ of24 days. The addition of 0.02% of thyme oil freshly prepared byhydrodistillation extended the T₃ to 35 days. Likewise, the alternativeaddition of 0.01% of ascorbyl palmitate extended the T₃ to 42 days.However, when both of these stabilisers were added simultaneously the T₃was extended to about 110 days indicating significant synergism. If theextension of the T₃ has taken place on an additive basis, a T₃ of about77 days at most would have been expected for the combination.

Example 4

Refined decolourised evening primrose oil with an initial PV of 1.8,which contained only 0.015 to 0.2% of residual total tocopherols, had aPV of 7.0 and a T₃ of 2 days in the open dish test. The addition of 0.1%thyme oil reduced the PV to 2.3 after 2 days and to 8.9 after 12 daysexposure. The further simultaneous addition of 0.005% of a highlypurified preparation of α-tocopherol suppressed the increase of PV evenmore effectively, to 2.1 after 2 days and 4.5 after 12 days exposure.

Example 5

A freshly rendered fish oil with an original PV of 10.6 oxidised veryrapidly in the open dish test, giving a T₃ of 5 days. The addition of0.02% of oregano oil extended the T₃ to 10 days and that of 0.05%ascorbyl palmitate to 14 days. However, the addition of both of thesesubstances simultaneously extended the T₃ to 50 days, demonstratingsubstantial synergism. If there had been no synergism, a T₃ of about 24days might have been expected.

Example 6

A concentrate of gamma-linolenic acid, "GLA 70" has an original PV of3.4. The concentrate contains 70% of GLA and no traces of nativetocopherols, and is therefore much more susceptible to oxidation thannatural products with smaller concentrations of the triply unsaturatedfatty acid. In the open dish test it afforded a T₃ of 10 days whichcould be extended to 25 days by the addition of 0.5% of a pepper oilprepared by steam distillation and to 17 days by the addition of 0.05%of delta-tocopherol. However, when both of these additives were presentsimultaneously, the T₃ increased to 65 in contrast to the expected muchshorter T₃ if the separate stabilisation effects of the two componentshad been merely additive.

Example 7

A concentrate of eicosapentaenoic acid, "EPA 50" containing 50% of thisquintuply unsaturated essential fatty acid in the free acid form and nonatural antioxidant immediately after manufacture, had a PV of 1.5. Inthe open dish test the extreme susceptibility of this polyunsaturatedconcentrate to oxidation was demonstrated in that after two days the PVwas over 100. However when 0.1% of clove oil and 0.05% ofalpha-tocopherol were added in the concentrate it was stabilised to suchan extent that the PV after 2 days was only 2.6 and after 7 days wasstill less than 5.10.

Example 8

Ethyl esters derived from an oil extracted from fungal biomass rich inarachidonic acid and having no detectable tocopherol content increasedrapidly in PV in the open dish test from an initial 3.4 to 75.5 after 3days. The same oil after the addition of 0.05% of thyme oil and 0.02% ofa commercially mixed tocopherol preparation had a 3-day PV of only 8.8under the same conditions.

We claim:
 1. A method of stabilizing naturally occurring polyunsaturatedoils comprising triply or more highly ethylenically unsaturated organicmaterials against ambient temperature atmospheric oxidation, whichmethod comprises adding to the polyunsaturated oil a combination ofessential oils each added in an amount of from 0.001 to 10% by weightrelated to the polyunsaturated oil, said essential oils consisting ofthe extract obtained by steam distillation of the vegetative parts ofthe oil-containing plant.
 2. The method according to claim 1, whereinthe essential oil is selected from the group consisting of thyme oil,oregano oil, pepper oil and clove oil.
 3. The method according to claim1 wherein each of said essential oils is added in an amount of from 0.01to 2% by weight related to the polyunsaturated oil.
 4. The methodaccording to claim 1 wherein each of said essential oils is added in anamount of from 0.05 to 1% by weight related to the polyunsaturated oil.5. The method according to claim 1, wherein a primary antioxidant isalso added to the polyunsaturated oil.
 6. The method according to claim5, wherein the primary antioxidant is added in an amount of from 0.001to 0.1% by weight related to the polyunsaturated oil.
 7. The methodaccording to claim 6, wherein the primary antioxidant is added in anamount of from 0.01 to 0.02% by weight related to the polyunsaturatedoil.
 8. The method according to claim 5, wherein the primary antioxidantis selected from the group consisting of tocopherol, tocotrienol,ascorbic acid and derivatives thereof.
 9. The method according to claim8 wherein said derivatives are lipid soluble derivatives.
 10. The methodaccording to claim 1, wherein the polyunsaturated oil comprises organicmaterials selected from the group consisting of gamma-linolenic acid,dihomo-gamma-linolenic acid and arachidonic acid.
 11. The methodaccording to claim 1, wherein the polyunsaturated oil comprises organicmaterials selected from the group consisting of eicosapentaenoic acid,docosapentaenoic acid and docosahexaenoic acid.
 12. The method accordingto claim 10 wherein the polyunsaturated oil is purified or refined. 13.The method according to claim 1 wherein the polyunsaturated oil has anincreased concentration of fatty acids.
 14. The pharmaceuticalcomposition, topical cosmetic preparation or nutritional supplement forhumans or animals comprising a polyunsaturated oil stabilized accordingto the method of claim 1.